Gaia Data Release 2. Summary of the contents and survey properties

Context. We present the second Gaia data release, Gaia DR2, consistingof astrometry, photometry, radial velocities, and information onastrophysical parameters and variability, for sources brighter thanmagnitude 21. In addition epoch astrometry and photometry are providedfor a modest sample of minor planets in the solar system. Aims: Asummary of the contents of Gaia DR2 is presented, accompanied by adiscussion on the differences with respect to Gaia DR1 and an overviewof the main limitations which are still present in the survey.Recommendations are made on the responsible use of Gaia DR2 results. Methods: The raw data collected with the Gaia instruments during thefirst 22 months of the mission have been processed by the Gaia DataProcessing and Analysis Consortium (DPAC) and turned into this seconddata release, which represents a major advance with respect to Gaia DR1in terms of completeness, performance, and richness of the dataproducts. Results: Gaia DR2 contains celestial positions and theapparent brightness in G for approximately 1.7 billion sources. For 1.3billion of those sources, parallaxes and proper motions are in additionavailable. The sample of sources for which variability information isprovided is expanded to 0.5 million stars. This data release containsfour new elements: broad-band colour information in the form of theapparent brightness in the GBP (330-680 nm) andGRP (630-1050 nm) bands is available for 1.4 billion sources;median radial velocities for some 7 million sources are presented; forbetween 77 and 161 million sources estimates are provided of the stellareffective temperature, extinction, reddening, and radius and luminosity;and for a pre-selected list of 14 000 minor planets in the solar systemepoch astrometry and photometry are presented. Finally, Gaia DR2 alsorepresents a new materialisation of the celestial reference frame in theoptical, the Gaia-CRF2, which is the first optical reference frame basedsolely on extragalactic sources. There are notable changes in thephotometric system and the catalogue source list with respect to GaiaDR1, and we stress the need to consider the two data releases asindependent. Conclusions: Gaia DR2 represents a major achievementfor the Gaia mission, delivering on the long standing promise to provideparallaxes and proper motions for over 1 billion stars, and representinga first step in the availability of complementary radial velocity andsource astrophysical information for a sample of stars in the Gaiasurvey which covers a very substantial fraction of the volume of our galaxy.